1. Field of the Invention
The present invention relates to a thermal image transfer recording method and a thermal image transfer recording medium that can be employed in the thermal image transfer recording method.
2. Discussion of Background
With reference to FIG. 2, when thermal transfer recording is performed by a printer provided with a conventional line plane thermal head 40, a thermal image transfer recording medium 30 and an image recording material 8 are tightly held between the thermal head 40 and a platen roller 50 and brought into pressure plane contact with each other as illustrated in FIG. 2.
In this thermal image transfer recording method, the thermal image transfer recording medium 30 comes into contact with the image recording material 8 and is then transported in close contact with the image recording material 8 in the direction of the arrow as illustrated in FIG. 2, and images are transferred to and recorded on the image recording material 8 by a heating element portion 401 of the thermal head 40, and the thermal image transfer recording medium 30 is then separated from the image recording material 8. In this process, a distance A between the point where image recording is started on the image recording material 8 by the heating element portion 401 and the point where the thermal image transfer recording medium 30 is separated from the image recording material 8 is generally as large as 3 to 15 mm. This distance is hereinafter referred to as the printing space distance A.
In this printing process, an ink in a thermal image transfer layer of the thermal image transfer recording medium 30 is melted by the thermal head 40 and then cooled in an insufficiently melted state before it is transferred to the image recording material 8, so that clear-cut transferred images are generally difficult to obtain on the image recording material 8.
More specifically, the temperature of the inside of the ink which is once melted is decreased during the transportation of the thermal image transfer recording medium 30 in the above-mentioned printing space distance A, so that occasionally the temperature of the ink almost reaches the ambient temperature thereof. Once the temperature of the ink decreases to such an extent, the ink usually aggregates because of its high inner cohesive force, so that it is extremely difficult to obtain images with high quality and high resolution by the transfer of the ink because of the formation of the so-called void images.
In order to solve this problem, thermal image transfer recording is recently performed by use of a printer provided with a line edge thermal head 20 as illustrated in FIG. 1.
In comparison with the line plane thermal head 40, the printing space distance A of the line edge thermal head 20 is as short as about 80 to 300 .mu.m, preferably about 150 to 250 .mu.m, so that the ink is transferred in a melted or softened state at high temperature and therefore the formation of void images can be avoided.
Furthermore, in the above-mentioned thermal image transfer recording, the difference in temperature between the image areas at high temperature and the non-image areas at low temperature is so large that sharp images with high resolution can be easily obtained.
However, there has been found a new problem in the line edge thermal head that improper transfer of a melted ink also takes place.
According to the analysis of this problem by the inventors of the present invention, this problem is considered to be caused as follows:
In this image recording process, in particular, when a rolled thermal image transfer recording medium is used, immediately after the ink is melted by the line edge thermal head, the recording medium is pulled in the take-up direction of the roll. Currently employed line edge thermal heads have a length in a range of about 5 to 26 cm, which is measured in the direction of the width of the rolled thermal image transfer recording medium, and this length may be increased in the future.
A currently employed thermal image transfer recording medium has almost the same width as the length of such a line edge thermal head.
Since the line edge thermal head is in line contact with the thermal image transfer recording medium, when the thermal image transfer recording medium is pulled in the transporting direction thereof, the tension applied to the thermal image transfer recording medium varies from point to point in the direction of the width thereof. The longer the line edge thermal head, the larger the scattering of the value of the tension applied to the thermal image transfer recording medium in the direction of the width thereof.
This scattering of the value of the tension is considered to cause not only the scattering of the temperature of the ink to be melted, but also improper image transfer.
Thus, the line edge thermal head has the problems that no ink transfer or uneven ink transfer takes place in normal operation, and improper ink transfer also takes place at a printing start point when printing speed is changed in a wide range from low speed to high speed.
Furthermore, in the thermal image transfer recording by use of the line edge thermal head 20 as shown in FIG. 1, the heating element portion 201 of the line edge thermal head 20 is in line contact with the thermal image transfer recording medium 30, so that high line pressure is applied to the thermal image transfer recording medium 30 and also to the image recording material 8. Therefore, when the thermal image transfer recording medium 30 and the image recording material 8 are held between the line edge thermal head 20 and the platen roller 50 and allowed to stand for a long period of time, there is caused a new problem that a thermal image transfer layer of the thermal image transfer recording medium 30 is brought into pressure contact with the image recording material 8 and transferred thereto, causing the smearing of the image recording material 8 with an ink in the transferred thermal image transfer layer.
Since the printer provided with such a line edge thermal head has been recently developed, and the above-mentioned problems have not been noticed until quite recently and countermeasures against such problems have not yet reported.